Molecular Gas and Star Formation in Dwarf Galaxies Alberto Bolatto Research Astronomer UC Berkeley Adam Leroy* Adam Leroy* Josh Simon* Josh Simon* Leo Blitz Leo Blitz * Hard working grad students

Why should you care? …Extreme properties are often sought for in Astronomy as one way to sharpen our understanding of fundamental concepts… Dwarf galaxies: are the first structures to form in bottom- up ΛCDM cosmologies are the first structures to form in bottom- up ΛCDM cosmologies have low heavy element abundances, just like primordial systems have low heavy element abundances, just like primordial systems are the simplest systems are the simplest systems Local dwarfs are windows onto the high-z Universe

A single-dish/interferometric survey MIDGet MIDGet A CO survey of IRAS-detected, compact, nearby, northern dwarf galaxies out to V LSR =1000 km s -1, with rotational velocities under ~100 km s -1 A CO survey of IRAS-detected, compact, nearby, northern dwarf galaxies out to V LSR =1000 km s -1, with rotational velocities under ~100 km s -1 Observed 121 central pointings with the Kitt Peak 12m Observed 121 central pointings with the Kitt Peak 12m Follow up of 30+ galaxies mapped using BIMA Follow up of 30+ galaxies mapped using BIMA Fabian Walters OVRO sample Fabian Walters OVRO sample UASO 12m BIMA

Two questions: What global properties distinguish galaxies with and without CO? What global properties distinguish galaxies with and without CO? Some of the best molecular gas predictors are surprising: L K, M dyn /L K, B-K (B-V) Some of the best molecular gas predictors are surprising: L K, M dyn /L K, B-K (B-V) Are there any differences between large and dwarf galaxies in their molecular gas/star formation properties? Are there any differences between large and dwarf galaxies in their molecular gas/star formation properties? Remarkably very few, even where some where expected Remarkably very few, even where some where expected

Distributions of detections/nondetections Best predictors of CO: L K, L B, Hubble Type,… Best predictors of CO: L K, L B, Hubble Type,… 1/5 Z 1/5 Z

Distributions of detections/nondetections Best predictors of CO: L K, L B, Hubble Type, FIR luminosity, B-K color, K-band mass to light ratio Best predictors of CO: L K, L B, Hubble Type, FIR luminosity, B-K color, K-band mass to light ratio

One of the best predictors of CO in the survey… M/L ~ 3 (B-band) and ~ 2 (K-band) M/L ~ 3 (B-band) and ~ 2 (K-band) But the correlation is much tighter at the low end in B light… CO nondetections are systematically fainter in K-band! But the correlation is much tighter at the low end in B light… CO nondetections are systematically fainter in K-band!

What is the driving relationship? L FIR, L K, L B, B-K, Hubble Type, Z, are all correlated L FIR, L K, L B, B-K, Hubble Type, Z, are all correlated Can we identify a driving parameter? Can we identify a driving parameter? Normalizing by L K removes trends and minimizes dispersion Normalizing by L K removes trends and minimizes dispersion MM M M

What is the driving relationship? L FIR, L K, L B, B-K, Hubble Type, Z, are all correlated L FIR, L K, L B, B-K, Hubble Type, Z, are all correlated Can we identify a driving parameter? Can we identify a driving parameter? Normalizing by L K removes trends and minimizes dispersion Normalizing by L K removes trends and minimizes dispersion M mol /L K is the tightest correlation. Across all galaxy sizes M mol /L K ~0.075 M mol /L K is the tightest correlation. Across all galaxy sizes M mol /L K ~0.075

What does it mean? Facts: Facts: Tightest M mol correlation is with L K, a proxy for M * and Σ * Tightest M mol correlation is with L K, a proxy for M * and Σ * Correlations with M gas (H I ) or M dyn are considerably weaker Correlations with M gas (H I ) or M dyn are considerably weaker Taken together, suggest that what matters in the H I H 2 conversion is the amount of matter in the disk (Σ * ), not just the amount of stuff Correlations with B-K could arise from enhanced photodissociation/less dust in bluer systems… Correlations with B-K could arise from enhanced photodissociation/less dust in bluer systems… …but systems with no CO tend to be underluminous (for their mass) in K-band, not overluminous in B-band …but systems with no CO tend to be underluminous (for their mass) in K-band, not overluminous in B-band Suggests that photodissociation plays only a secondary role in setting the global amount of H 2 This is indirect evidence in support of the local density (pressure) controlling H I H 2 This is indirect evidence in support of the local density (pressure) controlling H I H 2

Are large and dwarf galaxies different in their molecular gas/star formation properties?

The SFR vs. H 2 relationship… 1.4 GHz flux traces star formation (e.g., Condon et al. 2002, Murgia et al. 2002; SF SN CR synchrotron?) 1.4 GHz flux traces star formation (e.g., Condon et al. 2002, Murgia et al. 2002; SF SN CR synchrotron?)

The SFR vs. H 2 relationship… 1.4 GHz flux traces star formation (e.g., Condon et al. 2002, Murgia et al. 2002; SF SN CR synchrotron?) 1.4 GHz flux traces star formation (e.g., Condon et al. 2002, Murgia et al. 2002; SF SN CR synchrotron?) MIDGet and large galaxies fall on the same SFR - H2 correlation MIDGet and large galaxies fall on the same SFR - H2 correlation Σ SFR = ±0.1 Σ H2 1.3±0.1 Σ SFR = ±0.2 Σ H2 1.4±0.2

The SFR vs. H 2 relationship… is independent of Z! 1.4 GHz flux traces star formation (e.g., Condon et al. 2002, Murgia et al. 2002; SF SN CR synchrotron?) 1.4 GHz flux traces star formation (e.g., Condon et al. 2002, Murgia et al. 2002; SF SN CR synchrotron?) MIDGet and large galaxies fall on the same SFR - H2 correlation using the Galactic Xco! MIDGet and large galaxies fall on the same SFR - H2 correlation using the Galactic Xco!

Attempts to correct CO-H 2 for metallicity fail There is no segregation by inferred metallicity (using Richer & McCall 1995) There is no segregation by inferred metallicity (using Richer & McCall 1995)

Attempts to correct CO-H 2 for metallicity fail There is no segregation by inferred metallicity (using Richer & McCall 1995) There is no segregation by inferred metallicity (using Richer & McCall 1995) Corrections destroy the agreement! Corrections destroy the agreement!

Ways out of a constant Xco… Size-dependent corrections to RC-SFR (e.g. Bell 2003)? Size-dependent corrections to RC-SFR (e.g. Bell 2003)? Even then large changes in Xco are out of the question Even then large changes in Xco are out of the question A different SFR- H 2 regime for dwarf galaxies? A different SFR- H 2 regime for dwarf galaxies?

The sweet spot for star formation efficiency… A maximum star formation efficiency at M ? A maximum star formation efficiency at M ? To a first approximation galaxy-size / metallicity corrections to L FIR and Xco cancel To a first approximation galaxy-size / metallicity corrections to L FIR and Xco cancel A large Xco(Z) makes the maximum more pronounced A large Xco(Z) makes the maximum more pronounced

Summary M mol correlates very well with L K, not with M HI or M dyn M mol correlates very well with L K, not with M HI or M dyn Indirect support for a local density/pressure controlled H I H 2 transition Indirect support for a local density/pressure controlled H I H 2 transition Same SFR-H 2 relationship for dwarfs and large galaxies, suggesting constant CO-H 2 for star forming gas despite changing metallicity Same SFR-H 2 relationship for dwarfs and large galaxies, suggesting constant CO-H 2 for star forming gas despite changing metallicity A minimum H 2 depletion time / maximum SF efficiency at M ? A minimum H 2 depletion time / maximum SF efficiency at M ?

CARMA is moving forward